Excessive Nibbling May Interfere With Reproduction for the Noble Rhubarb

High in the Himalaya lives the noble rhubarb, Rheum nobile. It grows as a rosette before growing an infloresence up to one and a half metres tall. This is a final act of reproduction before it dies, what Alwyn Gentry referred to as a big bang form of seed production. The problem is that if all the effort goes into a final act, then herbivory before that can be devastating. Bo Song and colleagues have been looking at the demography of Rheum nobile to see what effect grazing is having on the plant.

Grazing is an issue and the problem is increasing in north-western Yunnan, China, where Song and colleagues conducted their study. “Yaks are highly relevant in Tibetan culture, especially regarding their function as a food source, i.e. for meat and milk,” write the authors. “Grazing areas visited by yaks may include R. nobile habitats, though this plant occurs in very remote areas. Chinese government policy to increase economic growth is in conflict with traditional land use and led to a greater yak and cattle population and an increase in grazing pressure… When collecting demographic data for this study, we therefore considered disturbances and grazing by yaks in the study sites, to assess their impact on demography and the long-term survival of the study populations.”

Demography allowed the team to identify what effect grazing had on the population as a whole. It also allowed them to estimate how long noble rhubarb needs to survive before it flowers, something that is currently unknown. When the plant does flower, it is extremely visible.

“In the year of flowering, a single stem several centimetres in diameter and of 1–1.5 m height is formed, with thousands of flowers covered by large translucent, yellow-whitish bracts forming a conspicuous tower-like inflorescence, attractive and visible from far away. The translucent bracts create a glasshouse effect and protect the reproductive organs from rain, mist, high UV radiation and low temperature,” write Song and colleagues.

In 2011. the team sampled two populations of at Huluhai and Yongjiongyi for a total of 741 plants. It is a lot of plants to keep track of, but as Song and colleagues explain in the paper, very necessary. “This large number of individuals was selected initially to ensure that in the three consecutive years of the census there would still be sufficient individuals of all sizes to calculate continuous demographic functions. The selected individuals were marked with wooden stakes with iron labels. In 2011 and three consecutive years (2012–2014), the number of leaves, the diameter of rosettes and the size of the largest leaf of all rosettes that survived (length and width) were measured.”

Analysis of both sites came to similar results. The time to flowering varied, with size being the biggest predictor of flowering. Once a plant got to a certain size, its survival from year to year was almost certain. On average, the team estimate it takes 33.5 years to reach flowering, making the long plant a long-player in reproduction. “Such a long delay of flowering is only affordable when the risk of mortality is low relative to the increase in seed production through further vegetative growth, and thus is a necessary pre-condition for the selection of perennial monocarpy,” write the scientists.

Overall, the strategy works for noble rhubarb. Or at least it did. Under normal conditions the population was set to expand slightly over time, but during the survey some plants went missing. The marks where some living plants were made it clear they were now inside a yak. As pastoralists and their yaks are pushed further into marginal lands, so they come more into contact with the rhubarb. Song and colleagues note the conflict over development at lower altitudes is having an effect higher up the slopes.

“Even in these remote sites, a balance between traditional land use and conservation needs is important. Conservation policies should consider that at remote sites in the Himalayas above 4000 m elevation, even moderate grazing by yaks is likely to pose a threat to the rare R. nobile, and this flagship species could be endangered by intensified traditional pastoralism.”